1. Field of the the Invention
This invention relates to improvements in the so-called plasma spray coating method and the apparatus therefor wherein a metal or a ceramic material is melted by means of a high-temperature plasma generated by electric arc, i.e. strong current through a gas, and is sprayed onto a substrate to form a strong coating film on a surface of substrate.
2. Description of the Prior Art
A plasma spray coating method and an apparatus therefor which have been broadly employed in the prior art are illustrated in FIG. 9 of the accompanying drawings. In the apparatus, a cathode 1 is held concentrically with a nozzle channel 25 of an anodic nozzle 2 by an insulator 12 so that the tip of the cathode may be placed near the entrance of the nozzle channel. Upstream, a plasma gas 8 is made to flow in via a charging port 7 for plasma gas.
The negative terminal of a power source 3 is connected to the cathode 1 by a conductor 5 and the positive terminal of the power source 3 is connected to the anodic nozzle 2 via an exciting power source 4 by a conductor 6. Reference numeral 6 depicts a cooling system. Usually, the anodic nozzle 2 has a double-walled structure (not shown) and the interior is arranged for being cooled always by a coolant, e.g. of soft water. When a D.C. voltage from a power source 3 is applied between the cathode and the anode and a high-frequency voltage is superposed by means of an exciting high-frequency power source 4 along with maintaining a flow of a plasma gas, usually an inert gas such as argon, through anodic nozzle 2 as shown by arrows 8 and 9, an electric arc 11 is generated from the tip of cathode 1 to the inner surface 105 of nozzle channel 25 of anodic nozzle 2. In this case, a short electric arc 11 tends to damage a wall 26 of nozzle channel 25, i.e. the inner wall of anodic nozzle 2. Accordingly, a large amount of plasma gas 8 is made to flow so that the generated electric arc may have as long a reach as possible within nozzle channel 25 to form an anode point 10 remote from the tip of cathode 1. Ths plasma gas flowing through nozzle channel 25 of anodic nozzle 2 is intensely heated to a high temperature by thus-formed arc 11, and jets out as so-called plasma state 16 from the forward end of anodic nozzle 2. Hereupon a spray coating material 18 is fed from a material charging pipe 17. The material is mixed with the plasma 16 of high temperature jetted from anodic nozzle 2, as shown by arrow 19, and forms instantly a molten material 20. Thus-formed molten material is sprayed onto a substrate 22 to form a coating film 21 thereon. In some cases, the spray coating material 18 from the material charging pipe 17 is fed at a point immediately before the outlet opening of anodic nozzle 2 or at a point immediately behind the outlet opening as shown by arrow 23.
In any cases of these plasma spray coating apparatuses employed in the prior art, an extremely large amount of gas is used for forming a long electric arc 11 within anodic nozzle 2, for preventing the erosion of wall 26 of nozzle channel 25, and for cooling the wall 26 of nozzle channel 25 by said plasma gas. The jetting speed of plasma gas leaving the outlet of anodic nozzle 2 is maintained at a very high value, usually in the range of Mach 05.-3.0. Due to this fact, a remarkably intense undesired sound of 110-120 phons is generated near the outlet opening of anodic nozzle 2. Therefore, plasma spray coating apparatuses of the prior art can be operated usually only in an isolated soundproof chamber. The operator cannot operate these plasma spray coating apparatuses without putting on a sound isolator. These are grave drawbacks in the prior art.
In addition, a plasma gas jetted from the outlet opening appears usually in the form of an extraordinarily bright flame. Thus, it is impossible to see directly said plasma gas. Accordingly, the operator of the apparatus is forced to put on ultraviolet protective goggles. On the other hand, usual plasma gases employable in plasma spray coating apparatus of the prior art are expensive inert gases, such as argon, helium and hydrogen. This is due to the fact that when a very active gas, such as air or oxygen, is used as plasma gas, the wall 26 of nozzle channel is oxidized to wear, especially at anode point 10, and the apparatus cannot be continuously operated for a long period of time. As these inert gases are expensive, a consumption of these gases in large amount for creating the high speed of the gas in said nozzle gives the disadvantage that the operating cost becomes quite high. In the prior plasma spray coating apparatus, the plasma gas 16 jetted from the front thereof forms an extremely turbulent flow because of remarkably high speed. Consequently, said gas flow involves a large amount of atmosphere near the jetting opening as shown by arrow 27. As a result, the temperature of plasma gas lowers rapidly. Thus, the conditions suitable for spray coating call for maintaining accurately the distance between the outlet opening of anodic nozzle 2 and the substrate 22. If the distance deviates from the accurate value, the shaping of the desired coating is quite difficult. In short, the quality control of the coating film requires a rigorous control of operational conditions. The quality control is achieved with difficulty.
Due to the situation detailed above, a large amount of high-speed gas is intensely blown against the substrate in the plasma spray coating apparatus of the prior art. Therefore, the substrate is limited to a material having high strength. Furthermore, no fine work can be performed.
One object of this invention is to provide a novel plasma spray coating apparatus wherein the drawback of the prior plasma spray coating apparatus which hinders the widespread use of the apparatus is removed.
Another object of this invention is to provide a plasma spray coating apparatus wherein the generation of an intense undesired sound is inhibited, the generation of a intense light including ultraviolet which allows no direct vision is inhibited, the extravagant consumption of expensive gas required for operation is saved, the control of operation conditions, such as the distance between the apparatus and the substrate, is not rigorous, the wear of parts is small, continuous operation can be achieved for a long period of time, a substrate having a relatively low strength can be worked, and fine work can be suitably performed.